The present invention relates to a scanning optical system.
Light beam scanners for deflecting a light beam with a light deflector to scan a surface are well known in the art particularly in association with laser printers, digital copying machines, laser facsimile receiver/transmitters, or the like.
A scanning optical system is incorporated in such a light beam scanner and positioned between the light deflector and the surface to be scanned by the deflected light beam. The scanning optical system is generally required to focus the deflected light beam onto the scanned surface (focusing function). The scanning optical system is also required to keep the scanned surface and a light beam deflecting point in substantially conjugate relationship in terms of geometrical optics with respect to an auxiliary scanning direction, for thereby correcting fluctuations of the light beam in the auxiliary scanning direction due to mechanical errors of the light deflector, which may be a rotating polygon, to stabilize the beam position in a main scanning direction that is perpendicular to the auxiliary scanning direction (facet error correcting function).
In general, scanning optical systems are anamorphic optical systems to perform both the focusing and facet error correcting functions. As is well known in the art, anamorphic configurations have heretofore been inexpensively achieved by elongate lenses such as cylindrical lenses, toroidal lenses, or the like. A scanning anamorphic optical system which deflects a light beam through a wide angle corresponding to a wide field angle cannot however correct curvature of field in the auxiliary scanning direction.
It has been proposed to correct curvature of field sufficiently by employing a modified cylindrical lens having a saddle-shaped lens surface (see Japanese Laid-Open Patent Publication No. 61-120112).
The above publication discloses a preferred form of modified cylindrical lens in which the ratio (d/f.sub.M) of the distance d along the optical axis between the saddle-shaped lens surface and a surface to be scanned to the focal length f.sub.M of the scanning optical system in a main scanning plane is smaller than 0.6. If d/f.sub.M .gtoreq.0.6, tolerances of the modified cylindrical lens would be too small to be practical. The publication only shows specific dimensional examples which meet the above ratio condition.